Welcome to the modding forums! Please read this thread for our (very short) list of guidelines and to see the (also very short) list of existing tutorials. Anything that's not covered by those tutorials is indeed either trial and error or copypasta from the vanilla game and other mods. But you can of course ask any questions you might have here on the forums or even request us to write a more extensive guide if you think that the modding community might profit from it! As for your questions:

Why are there squad and entity veterencies? I heard before about some glitch that wasn't patched for years... not sure if this is why? When for example, I want to change weapon accuracy for a squad vs change received accuracy, where would I have to put each veterency, and why?

Yes, that's the reason. Some veterancy modifiers were not applied to newly reinforces entities, making veterancy... not as useful as it should have been. I know that weapon accuracy was affected, maybe also cooldown and rate of fire, but I'm not quite sure about that. So these should be applied to entities, while received accuracy can be applied to squads. Basically, just look at how veterancy works for existing squads and try to copy it. If something is implemented in a certain way, there's usually a somewhat good reason for it.

Why do some squads have a scar function call? For example, when the grenadier vets up, there is a scar function call with function name XP1_Vet_Level2. What does this do? Why is it there? Why do some squads not have it? Also, is there somewhere where I can see a list of scar functions that are called? I can't seem to find it.

I'm not sure what it does... I'm not even sure it does anything. I have removed it for some units and didn't notice any effect, so you can probably safely delete it, if you want. It might be a leftover from campaign and ToW units, actually. There is a list of scar functions, but it's not complete and I didn't see this one anywhere.

Also, how does construction speeds work? Barbed wire has a time cost of 24. Is this time divided by the number of models building? So would a 4 model engineer take 6 seconds, and a 3 model engineer 8 seconds? If that's true, and I change a 4 model squad to be a 3 model squad, would I have to increase the construction speed modifier on the 3 model squad by 1.33 to achieve the same speed?

Thanks for giving my mod some spotlight again! As the creator, I would firstly like to say that I do not claim to be an expert on balance - I'm way too bad a player for that. But I would still like to encourage everyone to actually, properly read what this mod does before forming an opinion. Here's a link to the description.

This has several consequences:
- If you're just gonna garrison a mortar and let it auto-fire, you literally get no benefit from the pit. It's better to just put the mortar in green cover somewhere.
- Garrisoning two units in the pit means they will get obliterated by enemy artillery REALLY fast. They will take damage in the same way as, for example, Tommies in an ambient building.
- A pit with two mortars will cost you 100+240+240=580MP. And its auto-attack range will still be 80.
- Putting a land mattress in there will not let it magically auto-fire.

I absolutely fail to see how this would be OP. It's probably more of a nerf, to be honest. But with all that said, I don't think a change of this magnitude will ever enter scope. I just made this mod for everyone who's sick of playing with or against sim cities.

Which unit did you add to which building? I can think of two possible problems:

1. The unit you added has requirements that are not fulfilled. These could be found either in the entity or the squad, or possibly in both. Units that are normally called in by abilities usually have requirements for certain upgrades and CP values.

2. You chose the wrong building entity. For the Brits, these buildings would be correct:

Exactly. Alternatively, you could make a new ability, add it to the list in the 'army' category and then try to find a unique combination of upgrades granted by the commander. It works for some of them, but not for all. Here is an unfinished mod where I did that:

The 'commander' and 'commander_ability' categories can't be modified. Luckily, only the British faction actually uses that system properly, which is why you can modify other factions' commanders as I described here. For the Brits, the easiest way is to clone an exisiting ability with the same name and then completely change what it does and how it looks.